The present invention relates to the accurate reproduction of an image by a cathode ray tube (CRT), in general, and to a method and apparatus for readily and accurately making electrical adjustments to a CRT for the purpose of correcting defects in CRT reproduced images, in particular.
As the need for video display hard copy devices grows, so will the need for CRTs with increasingly tight tolerances on geometrical distortions of images reproduced by the CRT. This is particularly true in applications where a hard copy of a relatively complex design or set of mathematical functions displayed on a CRT is subsequently employed to further analyze the design or functions so displayed.
In one type of CRT geometry adjustment system, image size and centering are measured directly at the CRT by means of a template having a set of image-centering cross hairs formed therein that is placed directly on the CRT viewing screen. In a subsequent operation, distortion, linearity and image size at the CRT image plane are measured by employing a pattern generator to display a set of dots in the form of a grid pattern on the CRT viewing screen. The dots forming this type of grid pattern are then compared to a superposed reference target in the form of another set of dots formed on a transparent base material by focusing the viewing screen image onto the plane of the reference target mounted in said film holder. Deviations of the dots in the viewing screen displayed grid pattern from corresponding dots in the reference target are noted and then adjustments are made to correct such deviations and therefore the image defects that such deviations represent. A photograph of the compared grid-pattern images is also taken for further evaluation. This technique has several disadvantages. Among the disadvantages are that it is time-consuming, does not appear to be sensitive enough to measure extremely close linearity and distortion tolerances, requires the use of photographic film, is not comprehensive (separate templates are required for centering and image size correction) and it is not very accurate. In addition, separate image size adjustments must be made at the CRT viewing screen and subsequently at the reference target film plane when adjusting for distortion and linearity.
In another type of CRT geometry adjustment system, a grid of bullseye targets formed on a transparent base material is placed on or near the CRT viewing screen. A dot-type grid pattern is then displayed on the CRT viewing screen from a dot pattern generator. CRT geometry is adjusted by adjusting various component parts of the image-forming electron beam deflection system until every dot generated by the pattern generator falls in the center of its respective bullseye target. The amount by which a dot misses its bullseye is easily measured, and the bullseye rings are calibrated in percent error.
This bullseye technique has the advantage of widespread acceptance in the television industry. Reference targets for use with this technique are available commercially in many forms. The test is accurate, is sensitive enough to measure fairly close tolerances, does not require photographic film and is comprehensive (placing all dots within their respective bullseyes automatically adjusts for the proper degree or amount of distortion, linearity, centering, rotation and image size). However, adjusting CRT geometry by this bullseye technique consumes a relatively large amount of CRT test time in that each individual bullseye must be examined and/or considered for proper CRT adjustment.
A primary object of the present invention, therefore, is to provide a comprehensive method and apparatus for rapidly correcting defects in a CRT-reproduced image.
Another object of the present invention is to provide an improved method and apparatus for correcting defects in a CRT-reproduced image that is both accurate and sensitive to fairly close tolerance image defects.
A further object of the present invention is to provide an improved method and apparatus for correcting defects in a CRT-reproduced image that does not require the photographing of each CRT viewing screen as a part of the CRT adjustment process.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiment thereof, when taken in conjunction with the accompanying drawings.